It is known that there are a number of problems which are posed by attempts to provide an antirecoil or antireverse treatment on the running surface or bottom of a cross-country ski. This treatment should allow the bottom or running surface of the ski to have good sliding properties when the ski is slid forwardly on the snow, should not permit adhesion of snow to the underside of the ski, and should provide an antireverse characteristic whereby a reverse slipping of the ski is precluded or severely restricted. The term "reverse slip" and terms of similar import, such as "antirecoil" and "antibackslip" may be used from time to time herein to refer to a restriction on the backward movement of the ski caused by the character of the sole or running surface or parts thereof.
It has long been known that one can solve the problem of providing a resistance to reverse movement by the use of specific waxes selected in accordance with the characteristics of the snow over which the ski is to travel and requiring frequent renewal because of rapid wear of the wax surfaces. It is also known to provide the running surface of a ski with various formations such as ridges, fish-scale patterns or the like which frequently are only partially successful and which, in addition, are found to be noisy.
Certain other running surfaces of such skis for certain snow conditions require the local application of the antiadhesive products which it is necessary to renew from time to time and are designed to reduce the adhesion of snow to the running surface.
We have now found, after considerable research, that it is possible to provide an antirecoil sole or running surface for a cross-country ski which eliminates all of the problems enumerated above without requiring the sporadic application of specific products and which presents a polyvalence in the sense that it is applicable to all snow qualities and characteristics.
It is known, that the cross-country ski cycle in the classic advancing maneuver can be broken down essentially into three phases corresponding to the different interactions of the running surface of the ski and the snow. In the impulse or propulsion phase, the skier presses the ski against the snow in order to propel himself forwardly. In an intermediate phase or lightening phase, there is a sharp relief of pressure on the snow. This is generally followed in the last phase by a sliding of the ski along the snow.
We have studied the sliding mechanism and the interaction of the running surface and the ski on the snow and discovered that it is conditioned essentially by the polyphasic nature of the snow and that there are three factors involved in the participation of the snow in the mechanism of this interaction, namely, the hydrophobic character of the running surface with respect to the snow, the rheological characteristics of the surface of the underside of the ski and the topography of the surface of the sole of the ski.
The hydrophobicity of the running surface of the ski appears to be particularly important in the pressure reduction or lightening phase and during sliding. In the lightening phase, the hydrophobicity precludes adhesion of the snow which might otherwise tend to cause a phenomenon of balling-up of snow on the ski or of icing-up. By contrast, the hydrophobicity favors sliding whatever the nature of the snow and the mechanism causing it.
The mechanism of the rheological character is essentially the following:
(a) During sliding the contact with each grain of snow is sufficiently short so that the material of the sole or the treatment of the running surface is essentially elastic. The material is deformed only slightly and remains smooth and slidable.
(b) During the impulse generating or propulsion phase, when considerable pressure is applied by the skier against the snow, the contact with each grain of snow is sufficiently long for the material to have an essentially viscous character. The grains of snow can thus penetrate into the material and provide a resistance to reverse movement enabling the skier to push-off against the snow.
(c) When this interaction ceases, the material, thanks to its elastic memory, recovers its original shape and the ski is thus ready for its next cycle or step (propulsion phase).
These studies have shown that there is a difference in the tribological characteristics of the running surface of the ski in its propulsion and sliding phases. In the propulsion phase the treatment of the underside of the ski acts to resist reverse movement and the contact with this surface by the snow is an action of compression whereas during the sliding phase the contact with this layer involves a shearing action.
The propulsion phase is clearly much shorter than the sliding phase, i.e. of a duration of 0.1 to 0.2 seconds compared to 0.5 to 1.5 seconds.
We have found, considering the requirement for polyvalence, i.e. effective action on different types of snow, the thickness of the surface layer of the underside of the ski and the granulometry of the different type of snow, that the speed of deformation of the running surface during the brief propulsion period can vary between 0.1 and 10 sec. .sup.- 1 while, during the sliding phase, when the tangential velocity is of the order of 1 to 10 m/sec., the running surface is essentially subject to shear action and the velocity of the deformation is between 1 and 100 sec. .sup.-1, the velocity, of course, being in terms of an appropriate distance measure and being given without this means for the sake of comparison only. In order, therefore, to increase the anchorage phenomenon whereby the ski retards reverse movement during the propulsion phase, it is necessary to reduce the elasticity on compression while ensuring that the elasticity to shear will remain high during the sliding phase.